Organic Waste Recycling

Biofuels production 185

Table 4.17 Guidelines for the design of the gas-solids separator device for UASB reactor
(Lettinga and Hulshoff Pol 1992)

1. The slope of the settler bottom, i.e., the inclined wall of the gas collector, should be
   between 45-60°.


2. The surface area of the apertures between the gas collectors should not be smaller than
   15-20% of the total reactor surface area.


3. The height of the gas collector should be between 1.5-2 m at reactor heights of 5-7 m.


4. A liquid gas interface should be maintained in the gas collector in order to facilitate
   the release and collection of gas bubbles and to combat scum layer formation.


5. The overlap of the baffles installed beneath the apertures should be 10-20 cm in order
   to avoid upward flowing gas bubbles entering the settler compartment.


6. Generally scum layer baffles should be installed in front of the effluent weirs.


7. The diameter of the gas exhaust pipes should be sufficient to guarantee the easy
   removal of the biogas from the gas collection cap, particularly also in the case where
   foaming occurs.


8. In the upper part of the gas cap anti-form spray nozzles should be installed in the case
   where the treatment of the wastewater is accompanied by heavy foaming.

4.5 Biogas production

Rate of biogas yield per unit weight of organic wastes can vary widely
depending on the characteristics of influent feed and environmental conditions
in the digesters as stated in section 4.3. Data of biogas production from various
types of wastes can be obtained from literature or can be theoretically estimated
from chemical stoichiometry and kinetic reactions. This information is given
below.
A compilation of data of biogas yield from various organic waste materials is
presented in Table 4.21. The range of biogas production is 0.20-1.11 m^3 /kg of
dry solids, with CH 4 content being 57-69%. Theoretically, CH 4 production can
be determined according to the method outlined in Metcalf and Eddy Inc.
(2003):
During anaerobic digestion biodegradable organic matter (BODL) is covered
to mainly:

• CH 4 , CO 2 , NH 3 , H 2 and trace quantities of other gases, and


• Biological cells.